Attention

Magnetoencephalography detects variations in the brain’s magnetic field during various types of cerebral activity.

Photograph by BSIP / UIG via Getty

Every moment, our brains are bombarded with information, from without and within. The eyes alone convey more than a hundred billion signals to the brain every second. The ears receive another avalanche of sounds. Then there are the fragments of thoughts, conscious and unconscious, that race from one neuron to the next. Much of this data seems random and meaningless. Indeed, for us to function, much of it must be ignored. But clearly not all. How do our brains select the relevant data? How do we decide to pay attention to the turn of a doorknob and ignore the drip of a leaky faucet? How do we become conscious of a certain stimulus, or indeed “conscious” at all?

For decades, philosophers and scientists have debated the process by which we pay attention to things, based on cognitive models of the mind. But, in the view of many modern psychologists and neurobiologists, the “mind” is not some nonmaterial and exotic essence separate from the body. All questions about the mind must ultimately be answered by studies of physical cells, explained in terms of the detailed workings of the more than eighty billion neurons in the brain. At this level, the question is: How do neurons signal to one another and to a cognitive command center that they have something important to say?

“Years ago, we were satisfied to know which areas of the brain light up under various stimuli,” the neuroscientist Robert Desimone told me during a recent visit to his office. "Now we want to know mechanisms." Desimone directs the McGovern Institute for Brain Research at the Massachusetts Institute of Technology; youthful and trim at the age of sixty-two, he was dressed casually, in a blue pinstripe shirt, and had only the slightest gray in his hair. On the bookshelf of his tidy office were photographs of his two young children; on the wall was a large watercolor titled “Neural Gardens,” depicting a forest of tangled neurons, their spindly axons and dendrites wending downward like roots in rich soil.

Earlier this year, in an article published in the journal Science, Desimone and his colleague Daniel Baldauf reported on an experiment that shed light on the physical mechanism of paying attention. The researchers presented a series of two kinds of images—faces and houses—to their subjects in rapid succession, like passing frames of a movie, and asked them to concentrate on the faces but disregard the houses (or vice versa). The images were “tagged” by being presented at two frequencies—a new face every two-thirds of a second, a new house every half second. By monitoring the frequencies of the electrical activity of the subjects’ brains with magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI), Desimone and Baldauf could determine where in the brain the images were being directed.

The scientists found that, even though the two sets of images were presented to the eye almost on top of each other, they were processed by different places in the brain—the face images by a particular region on the surface of the temporal lobe that is known to specialize in facial recognition, and the house images by a neighboring but separate group of neurons specializing in place recognition.

Most importantly, the neurons in the two regions behaved differently. When the subjects were told to concentrate on the faces and to disregard the houses, the neurons in the face location fired in synchrony, like a group of people singing in unison, while the neurons in the house location fired like a group of people singing out of synch, each beginning at a random point in the score. When the subjects concentrated instead on houses, the reverse happened. Furthermore, another part of the brain, called the inferior frontal junction, a marble-size region in the frontal lobe, seemed to conduct the chorus of the synchronized neurons, firing slightly ahead of them. Evidently, what we perceive as “paying attention” to something originates, at the cellular level, in the synchronized firing of a group of neurons, whose rhythmic electrical activity rises above the background chatter of the vast neuronal crowd. Or, as Desimone once put it, “This synchronized chanting allows the relevant information to be ‘heard’ more efficiently by other brain regions.”

A connection between attention and neural synchrony was hypothesized by Ernst Niebur and Chrisof Koch twenty years ago. Desimone was one of the first scientists to prove it for particular cases, in 2001. A pioneer in the field, he is quick to mention other leaders, such as John Reynolds of the Salk Institute, in La Jolla, California, who uses a combination of physics, neurophysiology, and computational neural modelling to study how objects in the visual field, such as separate highlighted areas in an illuminated grid, compete with each other for attention. Meanwhile, Sabine Kastner of Princeton has recently begun comparing humans with monkeys in their attention to visual tasks; and Columbia’s Michael Goldberg has recently shown that, in the process of attention, a particular area of the brain called the lateral parietal area “sums up” visual signals and cognitive signals. In this growing field of neuroscience, Desimone has personally trained more than thirty-five people.

I asked Desimone how the conductor of the neuronal chorus—in this case, the inferior frontal junction—knows when a particular stimulus should be attended to. In his experiment, the subjects were told to focus their attention on either faces or houses, but what about an unexpected stimulus—say, a charging lion, or the sudden entrance of an attractive celebrity? “We don’t understand the answer to that yet,” Desimone said. And how do disparate voices come into synchrony? Can they do so merely by exchanging information among themselves, or do they need an outside director? At the second question, Desimone broke out in a boyish grin and took six small metronomes from his briefcase. He placed them side by side on a wooden board, balanced on two empty lemon-soda cans. Then he set the metronomes ticking, out of synch with one another. After a couple of minutes, they were all ticking in synchrony. They had communicated with one another and come into synch solely through the side-to-side movement of the board, without any outside agency. Neurons, of course, use a different method of communication: passing chemical messengers between the hundreds of filaments radiating from each neuron. Desimone’s pendulums suggest that some neurons could come into synch on their own, without a conductor. But neuroscientists don't yet know which neuronal processes are self-organizing and which require a higher-level cognitive director.

As my visit came to an end, I asked Desimone about the strange experience of consciousness, to me the most profound and troubling aspect of human existence. How does a gooey mass of blood, bones, and gelatinous tissue become a sentient being? How does it become aware of itself as a thing separate from its surroundings? How does it develop a self, an ego, an “I”? Without hesitation, Desimone replied that the mystery of consciousness was overrated. “As we learn more about the detailed mechanisms in the brain, the question of ‘What is consciousness?’ will fade away into irrelevancy and abstraction,” he said. As Desimone sees it, consciousness is just a vague word for the mental experience of attending, which we are slowly dissecting in terms of the electrical and chemical activity of individual neurons. As an analogy, he said, consider a careering automobile. A person might ask: Where inside that thing is its motion? But the viewer would no longer ask that question after he understood the engine of the car, the manner in which gasoline is ignited by spark plugs, the movement of piston and crankshaft.

I am a scientist and a materialist myself, but I left Desimone’s office feeling bereft. Although I cannot say exactly why, I do not want my thoughts, my emotions, and my sense of self reduced to the electrical tinglings of neurons.

I prefer that at least some parts of my being remain in the shadows of mystery. I think of a comment by Einstein: “The most beautiful experience we can have is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science.”